S. aureus is a potent, opportunistic human pathogen that has evolved in a symbiotic relationship with its hosts and is notorious for its ability to caue life-threatening diseases such as sepsis, pneumonia and endocarditis. In fact in 2005 S. aureus was responsible for more death in the US than any other microbial pathogen. S. aureus is unique in that the organism produces over a dozen fibrinogen (Fg) binding cell wall anchored proteins (MSCRAMMs) or secreted proteins (SERAMs). Many of these proteins act as potent virulence factors and can recruit Fg and assemble a Fg containing coat surrounding and protecting the bacteria from phagocytosis and clearance. We believe that this Fg containing shield represents a key to understand the unique features of S. aureus virulence including the organism's demonstrated resistance in several active and passive vaccination trials. Consequently we propose to characterize the shield inducing staphylococcal proteins and their interactions with Fg. We will use X-ray crystallography of complexes formed between the bacterial proteins and intact Fg or Fg fragments complemented by extensive biochemical studies of the Fg interactions. Preliminary results show that the MSCRAMMs use a combination of a primary and a secondary synergistic site to bind Fg with high affinity whereas a common linear Fg binding motif present in two of the SERAMs exhibits an amazingly high affinity for Fg. Our proposed studies will identify the interactive surfaces of the proteins as well as conformational changes induced in Fg upon binding to the staphylococcal proteins. Based on the detailed information of staphylococcal protein/Fg interactions, we will be able to identify MSCRAMM variants with altered affinity for Fg and explore the possibility that this translates to altered virulence in a septicemia mouse model. Finally we will use our detailed understanding of staphylococcal Fg interactions to generate mAbs that inhibit Fg binding to MSCRAMMs and SERAMs and in the future could be developed to useful therapeutic agents.